7^^ A VERS. [Vol. VI. 



one thing and has as great a curvature as some species of Birds. 

 The importance of this spiral twisting is due to the fact that 

 the Alligator belongs to a group of animals possessing all grades 

 of development of the cochlear pocket or tube, as the case may 

 be. The lower members of the reptilian class have a lagenar 

 pocket not much advanced beyond the Torpedo condition, while 

 the highest families of the class show a greater differentiation 

 of the cochlear tube with its contained organs than the Birds. 

 Thus in the Saurians the cochlear tube begins a spiral twisting 

 that is carried on through the whole of the mammalian group. 

 The homologies of the cochlear framework I cannot enter into 

 in this place. The roof of the cochlear tube acquires such 

 relations to the bounding walls and is so sharply marked off 

 from the side walls in some parts as to form a membrana 

 Reissneri with nearly mammalian characteristics. 



The membrana basilaris is composed of three superposed 

 layers of fine, parallel fibres, packed closely together in the 

 form of three continuous sheets, of which the middle one is the 

 best developed. The membrane itself is not pierced by the coch- 

 lear nerve as in Mammals, for the nerve enters the membrane 

 after piercing the cartilaginous frame of the inner edge, and is 

 then distributed to the different parts of the cochlear organ by 

 radiation to the inside and outside from a point in the floor 

 near the middle of the sensory structures. In its thickest parts 

 the membrana basilaris is thicker than the remaining parts of 

 the cochlear organ, and is suited in nowise as a vibrating mem- 

 brane (PL X, Fig. 3). The fibres of the middle layer cross 

 from one side to the other of the framework, and in the thickest 

 parts of the membrane present a very characteristic appearance, 

 as indicated in PI. VII, Fig. 2 ; and PI. X, Fig. 3. 



Retzius thought that only the middle epithelial ridge of the 

 cochlea was concerned in the auditory function as a nerve-end 

 organ. This he called without hesitation the papilla acustica 

 basilaris, and considered it to be the homologue of the organ 

 of Corti. In this, however, he is in error. Besides the very 

 important organ he recognized, there is another, nearly as large, 

 lying to the inner side of the incipient organ of Corti, which 

 Retzius has several times imperfectly figured. He has likewise 

 correctly given the relation of the membrana tectoria to it, as 

 seen in ordinary preparations of the Alligator ear. This other 



